The Debate on synchronized(this) in Java: When to Use Private Locks

Introduction

In Java multithreading, synchronization is crucial for thread safety. The use of synchronized(this) as a common synchronization method often sparks debate: some advocate avoiding it in favor of private object locks. This article, based on Stack Overflow discussions, particularly high-scoring answers, analyzes the core issues and provides practical guidance.

Semantics and Equivalence of synchronized(this)

synchronized(this) is semantically equivalent to declaring a method as synchronized. For example:

public void method() {
    synchronized (this) {
        // critical section code
    }
}

This is equivalent to:

public synchronized void method() {
    // critical section code
}

This equivalence means synchronized(this) uses the current instance as a lock, controlling concurrent access to instance methods. However, explicit synchronized(this) blocks allow non-synchronized operations outside the lock, which can introduce complexities like the double-checked locking problem.

Analysis of Controversial Points

Lock Exposure

A primary argument against synchronized(this) is that the lock might be "stolen" or accidentally used. Since this is publicly accessible, external code could synchronize on the same object and interfere. For example:

// External code might do this:
synchronized (yourObject) {
    // This could block your methods
}

However, the top answer notes this is more about "accidental" than "malicious" issues. synchronized(this) should be considered part of the class interface and documented. In some cases, like Collections.synchronizedMap, allowing external lock use is by design.

Throughput Impact

Another common view is that all synchronized methods sharing the same lock (i.e., this) reduce throughput. But simply replacing it with a private lock doesn't automatically improve performance. For instance, if two methods modify different variables:

public void method1() {
    synchronized(this) {
        a++;
    }
}

public void method2() {
    synchronized(this) {
        b++;
    }
}

Even though a and b have no data race, threads still wait for the same lock, potentially reducing concurrency. Optimizing throughput requires finer-grained lock design, not just avoiding synchronized(this).

Information Exposure

Using synchronized(this) may unnecessarily expose lock information, related to lock exposure. If a class doesn't need external synchronization, a private lock can hide implementation details as a defensive programming measure.

Advantages and Scenarios for Private Locks

Private locks use internal objects for synchronization, e.g.:

private final Object lock = new Object();

public void method() {
    synchronized(lock) {
        // critical section code
    }
}

This avoids lock exposure and allows fine-grained control. As supplementary answers show, when different methods operate on independent data, multiple private locks can enhance concurrency:

private final Object lockA = new Object();
private final Object lockB = new Object();

public void method1() {
    synchronized(lockA) {
        a++;
    }
}

public void method2() {
    synchronized(lockB) {
        b++;
    }
}

This reduces unnecessary thread blocking but increases code complexity.

When to Use synchronized(this)

The top answer emphasizes that synchronized(this) is a safe, widely-used, and well-understood idiom. It may be appropriate in cases such as:

• Class design requires external synchronization, like in Collections.synchronizedMap.
• Synchronization needs are simple, with all methods sharing one lock being sufficient and no performance bottlenecks.
• It's already prevalent in the codebase, and consistency is more important.

As noted in Effective Java, instance locks are typical, but exceptions exist. Avoiding synchronized(this) should not be a blanket rule due to debugging difficulties.

Conclusion

The choice between synchronized(this) and private locks depends on context. synchronized(this) suits simple synchronization and cases needing external lock access, while private locks are better for hiding implementation, fine-grained control, or avoiding accidental contention. Developers should decide based on class interface design, performance needs, and code clarity, rather than following absolute rules. In multithreading, understanding lock semantics and impacts is key, and regardless of approach, thread safety and proper documentation are essential.